security: P0 fixes - SQL injection prevention + data race fix

- Add identifier validation for database names in PostgreSQL and MySQL
  - validateIdentifier() rejects names with invalid characters
  - quoteIdentifier() safely quotes identifiers with proper escaping
  - Max length: 63 chars (PostgreSQL), 64 chars (MySQL)
  - Only allows alphanumeric + underscores, must start with letter/underscore

- Fix data race in notification manager
  - Multiple goroutines were appending to shared error slice
  - Added errMu sync.Mutex to protect concurrent error collection

- Security improvements prevent:
  - SQL injection via malicious database names
  - CREATE DATABASE `foo`; DROP DATABASE production; --`
  - Race conditions causing lost or corrupted error data

internal/database/mysql.go

@@ -126,13 +126,46 @@ func (m *MySQL) ListTables(ctx context.Context, database string) ([]string, erro
 	return tables, rows.Err()
 }
 
+// validateMySQLIdentifier checks if a database/table name is safe for use in SQL
+// Prevents SQL injection by only allowing alphanumeric names with underscores
+func validateMySQLIdentifier(name string) error {
+	if len(name) == 0 {
+		return fmt.Errorf("identifier cannot be empty")
+	}
+	if len(name) > 64 {
+		return fmt.Errorf("identifier too long (max 64 chars): %s", name)
+	}
+	// Only allow alphanumeric, underscores, and must start with letter or underscore
+	for i, c := range name {
+		if i == 0 && !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_') {
+			return fmt.Errorf("identifier must start with letter or underscore: %s", name)
+		}
+		if !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_') {
+			return fmt.Errorf("identifier contains invalid character %q: %s", c, name)
+		}
+	}
+	return nil
+}
+
+// quoteMySQLIdentifier safely quotes a MySQL identifier
+func quoteMySQLIdentifier(name string) string {
+	// Escape any backticks by doubling them and wrap in backticks
+	return "`" + strings.ReplaceAll(name, "`", "``") + "`"
+}
+
 // CreateDatabase creates a new database
 func (m *MySQL) CreateDatabase(ctx context.Context, name string) error {
 	if m.db == nil {
 		return fmt.Errorf("not connected to database")
 	}
 
-	query := fmt.Sprintf("CREATE DATABASE IF NOT EXISTS `%s`", name)
+	// Validate identifier to prevent SQL injection
+	if err := validateMySQLIdentifier(name); err != nil {
+		return fmt.Errorf("invalid database name: %w", err)
+	}
+
+	// Use safe quoting for identifier
+	query := fmt.Sprintf("CREATE DATABASE IF NOT EXISTS %s", quoteMySQLIdentifier(name))
 	_, err := m.db.ExecContext(ctx, query)
 	if err != nil {
 		return fmt.Errorf("failed to create database %s: %w", name, err)
@@ -148,7 +181,13 @@ func (m *MySQL) DropDatabase(ctx context.Context, name string) error {
 		return fmt.Errorf("not connected to database")
 	}
 
-	query := fmt.Sprintf("DROP DATABASE IF EXISTS `%s`", name)
+	// Validate identifier to prevent SQL injection
+	if err := validateMySQLIdentifier(name); err != nil {
+		return fmt.Errorf("invalid database name: %w", err)
+	}
+
+	// Use safe quoting for identifier
+	query := fmt.Sprintf("DROP DATABASE IF EXISTS %s", quoteMySQLIdentifier(name))
 	_, err := m.db.ExecContext(ctx, query)
 	if err != nil {
 		return fmt.Errorf("failed to drop database %s: %w", name, err)